1. Field of the Invention
The invention relates to the field of flexible wiring boards, particularly to a process for manufacturing a flexible wiring board capable of forming fine metal bumps and the flexible wiring board manufactured thereby.
2. Description of Related Art
Recently, there is an increasing demand for miniaturized semiconductor devices and a great importance is placed on flexible wiring boards on which a bare-chip semiconductor can be mounted.
FIGS. 4(a)-(d) is a processing diagram showing a process for manufacturing a flexible wiring board of the related art. Referring to the processing diagram, the process is explained in order. At first, a copper foil is applied on a polyimide film 111 and then the copper foil is patterned into a copper wiring 112 (FIG. 4(a)).
Then, the surface of polyimide film 111 is irradiated with laser beam 114 (FIG. 4(b)) to form openings 115 having a predetermined diameter (FIG. 4(c)). At this stage, the top surface of copper wiring 112 is exposed at the bottoms of openings 115, and then copper wiring 112 is plated with copper while the bottom surface is protected with a resin film 117 so that copper grows in openings 115 to form metal bumps 116.
When a bare-chip semiconductor device is to be mounted on such a flexible wiring board 110, an anisotropic conductive film is applied on metal bumps 116 and bonding pads of the semiconductor device are brought into contact with metal bumps 116 via the anisotropic conductive film and pressure is applied. Then, circuits within the semiconductor device contact copper wiring 112 via the anisotropic conductive film and metal bumps 116.
Flexible wiring boards of this type 110 are recently much used because they are thin and light and freely foldable to provide a high mounting flexibility.
However, residues of polyimide film 111 remain on the top surface of metal wiring 112 exposed at the bottoms of openings 115 when openings 115 are formed with laser beam 114 as described above. Residues are removed by immersing the assembly in a chemical solution after openings 115 have been formed. However, it becomes more difficult for the chemical solution to penetrate into openings 115 as openings 115 become finer, and therefore more difficult to remove residues.
If residues cannot be removed, copper deposition speed varies from opening 115 to opening 115, whereby homogeneous metal bumps 116 cannot be formed.
Another problem is variation in the diameter of fine openings 115 (about 0 μm to 50 μm) formed by irradiating a rigid polyimide film 111 with laser beam 114, resulting in variation in the diameter and height of metal bumps 116 which causes failure of connection with semiconductor chips.
Still another problem is that it is difficult to reduce the spot diameter of high power laser beam 114, which makes it impossible to form openings 115 having a diameters smaller than 40 μm, contrary to the recent demand for finer openings 115.